1. Field of the Invention
This invention relates to a constant flow control device for a pressure pot shot peening machine. Such structures of this type, generally, assure that the air flow and the pressure drop through the pressure pot shot peening nozzle remain independent of uncontrolable upstream conditions, thus assuring constant velocity of the shot stream regardless of shot delivery hose routing or wear.
2. Description of the Related Art
A key control difficulty inherent in conventional pressure pot shot peening machines involves maintaining constant conditions of the shot stream with a remote system of control. That is, pressure pot shot machines are normally set up and controlled as shown in FIG. 1. A constant pressure supply of air is mixed with shot well upstream of the shot peening nozzle. The shot mass flow rate of the stream is controlled by flow through a fixed orifice or by a closed loop mass flow controller such as a magnetic flow controller built by Electronics, Inc. To assure shot flow stability, a bleed line (see FIG. 1) maintains a constant pressure across the shot flow controller.
The need for a remote system of control is directed by requirements of machine motion and flexibility and the hostile environment of the shot peening cabinet itself. Thus, the cabinet containing various control functions described above must be well removed from the shot delivery nozzle. The shot delivery hose, usually about 20 feet long, is therefore needed to connect the cabinet to the nozzle. The air velocity in the hose must be sufficient to sweep the shot along with it to the nozzle where the two phase flow is accelerated and ejected from the nozzle at a velocity sufficient to perform the peening operation.
The problem with pressure pot systems is that there is a sizable pressure drop in the delivery hose, as well as, the nozzle itself due to the shot/air stream. Thus, the pressure available in the throat of the nozzle where the shot is accelerated to its terminal velocity is well below the pressure control set point. Worst yet, the pressure drop is not constant. It depends upon the routing of the hose, hose wear, nozzle geometry, and the mass flow rate of the shot. There is no better illustration of this fact than the well known observation that the peening intensity increases for otherwise fixed conditions when the hose wears, because the reduced flow resistance makes more pressure available at the nozzle. Therefore, a more advantageous system, then, would be presented if the air flow and the pressure drop through the nozzle remained independent of uncontrolable upstream conditions.
It is apparent from the above that there exists a need in the art for a pressure pot shot peening machine which is capable of delivering shot to the workpiece, and which at least equals the shot peening characteristics of the known shot peening machines, but which at the same time is capable of assuring that the air flow and the pressure drop through the nozzle remains independent of uncontrollable upstream conditions. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.